The present invention relates generally to a lubricating oil composition that is represented by refrigerating machine lubricating oil, viscous coupling lubricating oil, gear oil, mechanical booster pump oil, shock absorber oil, turbo-molecular pump bearing oil and belt tensioner oil and is excellent in stability to hydrolysis, heat and oxidation as well as in lubricating properties and, more particularly, to a refrigerating machine lubricating oil composition that is excellent in stability to hydrolysis, heat and oxidation as well as in lubricating properties, and is well compatible with a non-chlorine type of fluorine-containing refrigerant.
So far, chlorine-containing refrigerants such as R11 (CCl.sub.3 F), R12 (CCl.sub.2 F.sub.2), R123 (CF.sub.3 CHCl.sub.2) and R22 (CHClF.sub.2) have been used as refrigerants for refrigerating machinery. In recent years in which the development of substitute flon is in urgent need in view of environmental problems, however, non-chlorine type flourine-containing refrigerants such as 1.1.1.2-tetrafluoroethane (R134a), difluoromethane (R32) and 1.1.2.2.2-pentafluoroethane (R125) have attracted wide attention. In has also been proposed to use as refrigerating machine oil polyalkylene glycol or ester oils that are compatible with these refrigerants (R134a, R32, R125, and so on). As the efficiency of refrigerating machinery increases, such refrigerating machine oil is now required to have an increased heat stability, and ester or polyalkylene glycol oils that is excellent in stability are used to this end. However, these ester or polyalkylene glycol oils are still less than satisfactory, because they hydrolyze in the presence of small amounts of water or air, or oxidize, resulting in an increase in the acid number. Their stability increase may be achieved by the incorporation of an epoxy compound in them, but the resulting oils become insufficient in terms of compatibility with refrigerants or stability, although varying depending on the structure of epoxy.
In the case of conventional chlorine-containing refrigerants, there is no need of taking any special care of their lubricating properties, because they possess some lubricating properties by themselves. However, non-chlorine type fluorine-containing refrigerants are required to be increased in lubricating properties for lack of lubricating properties. It has been known to incorporate a lubricant such as tricresyl phosphate in refrigerating machine lubricating oil, but this offers a problem that the resulting lubricating oil fails to produce its own lubricating properties sufficiently, when actually used with a non-chlorine type of fluorine-containing refrigerant.
A general object of the invention is to provide a lubricating oil composition that is more excellent in stability to hydrolysis, heat and oxidation as well as in lubricating properties, and a particular object of the invention is to provide a refrigerating machine lubricating oil composition used with a non-chlorine type of fluorine-containing refrigerant, which is more excellent in stability to hydrolysis and heat, esp., oxidation, as well as in lubricating properties, and which is more excellent in compatibility with the refrigerant.